This invention relates to a method of charging batteries connected in series, and in particular to a method which charges while balancing the voltage of each battery.
Battery packs with a plurality of batteries connected in series are used in various applications. In particular, since output voltage can be increased by increasing the number of batteries connected in series, this type of battery pack is suitable for applications requiring large output. However, when many series-connected batteries are charged, they gradually become unbalanced even though the same current flows in all batteries. This is because electrical characteristics and environmental conditions during charging and discharging cannot be maintained uniformly over all batteries (e.g. temperature variation). There is a strong tendency for a battery, which has reduced capacity, to over-charge or over-discharge relative to other batteries when connected in series. This is directly related to an observable reduction in total capacity of a series-connected battery pack.
To eliminate this drawback, a method was developed which connects discharge circuits for the series connected batteries and balances battery voltage via these discharge circuits (See, for example Japanese Patent Application HEI 5-49181).